How much of our lives and the things that happen to us are we responsible for? The common-sense answer would be that we are responsible only for those actions which we cause and intend to cause, but that we are not responsible for the things that happen to us, good or bad, that just happen to us. That is to say, where the causes was something other than us. For example, things caused by society, luck, accident, other people, or physics. And this common-sense answer is often taken as relinquishing us from responsibility for things like being drafted in a war, or being born into poverty, or indeed being born into wealth.
But in Being and Nothingness, Sartre takes these ideas and arrives at a completely different and much more extreme conclusion. We are wholly responsible for ourselves, he argues, including even if we are drafted into a war.
Sartre argues that there could be no fact in the world that could motivate our conscious actions by itself. We would always need in addition goals, values and interpretations in order to have the intentions that lead to our deliberate actions. Put another way, for a cause (some fact in the world) to motivate an action it must be experienced as a cause. So for example, for Bill to be motivated to perform the action of getting a drink because of the cause 'thirst', he must first experience thirst as as a cause. He must have ideas about what one should do if they are thirsty. If he did not have the idea that when thirsty he should get a drink, on the other hand then thirstiness would cease to be a cause for him doing so.
So similarly, according to Sartre's thinking, even if Bill is stranded on a dessert island and is thirsty and can't get a drink, it's his responsibility, because it's he, not his thirst, that has motivated him to want a drink.
Sartre himself gives the example of war.
"If I am mobilized in a war, this war is my war... I deserve it first because I could always get out of it by suicide or by desertion; these ultimate possibles are those which must always be present for us when there is a question of envisaging a situation. For lack of getting out of it, I have chosen it. ... If therefore I have preferred war to death or to dishonour, everything takes place as if I bore the entire responsibility for this war. of course others have declared it, and one might be tempted perhaps to consider me as a simple accomplice. But...it depended on me that for me and by me this war should not exist, and I have decided that it does exist... But in addition the war is mine because by the sole fact that it arises in a situation which I cause to be, and that I can discover it there only by engaging myself for or against it"